In a variety of equipment such as a copying machine, a printer and a liquid crystal display device, a surface potential of a specific portion in the equipment is measured. For example, as a photosensitive drum is used repeatedly in an electrostatic copying machine, a charged voltage of the photosensitive drum gradually decreases. When the charged voltage of the photosensitive drum falls below a certain level, a latent image quality deteriorates and thus less toner is applied or color unevenness occurs, leading to deterioration in printing quality. Therefore, a charged voltage of the photosensitive drum after charged by a charger has hitherto been measured by a surface potential sensor to monitor a deterioration state and a product cycle of the photosensitive drum. Then, when the charged voltage of the photosensitive drum falls below a predetermined value, for example, a warning that the photosensitive drum should be exchanged is given.
(Chopper-Type Potential Sensor)
There are several kinds of surface potential sensors, and one of them is a chopper-type surface potential sensor. FIG. 1 is a schematic diagram for explaining a measurement principle of chopper-type surface potential sensor 11. Surface potential sensor 11 has detection electrode 13 inside casing 12, and detection electrode 13 is connected to a ground via resistor Rs. In casing 12, window 14 is open as opposed to detection electrode 13. Further, chopper 15 is provided between detection electrode 13 and window 14. Chopper 15 is held at a ground potential, and has a pair of shields 16 that shield an electrostatic field passing through window 14 and moving toward detection electrode 13. Shields 16 vibrate in a fixed cycle, and a space between shields 16 is cyclically opened and closed. When surface potential sensor 11 is installed with window 14 facing charged object 17, an electrostatic field that comes out of charged object 17 is incident on the inside of surface potential sensor 11 from window 14, and further passes through between shields 16 to reach detection electrode 13. However, since shields 16 are cyclically opened and closed, an incidence area (number of electric force lines) of the electrostatic field that reaches detection electrode 13 changes.
This surface potential sensor 11 uses an electrostatic induction phenomenon. When detection electrode 13 is applied with an electrostatic field with strength Eo (proportional to charging potential Vo of charged object 17) from charged object 17, induction charge q is generated in detection electrode 13. However, since this cannot be detected as an electrical signal in such a static state, chopper 15 is provided between charged object 17 and detection electrode 13, and by opening and closing chopper 15, the incidence area of the electrostatic field is cyclically changed. When the incidence area of the electrostatic field is cyclically changed by chopper 15, induction charge q is cyclically changed in the same manner, and displacement current Is flows from detection electrode 13 to the ground. This displacement current Is is converted to alternating current voltage signal Vs by resistor Rs. Then, by measuring alternating current signal Vs, charging potential Vo of charged object 17 can be sensed. As such a chopper-type surface potential sensor, for example, one disclosed in Patent Document 1 is known.
However, in such a chopper-type surface potential sensor, a chopper for cyclically changing an incidence area of an electrostatic field is essential. Since a mechanical actuator is required in order to open and close the chopper, it is difficult to miniaturize the surface potential sensor. For example, in the surface potential sensor disclosed in Patent Document 1, a tuning fork-type component is required as shown in FIG. 15 of Patent Document 1. This tuning fork-type component is one formed by providing a pair of shields in its leading end portion and fitting a piezoelectric vibrator to its base portion, and is designed to amplify (resonate) vibration of the piezoelectric vibrator by means of the tuning fork-type component to open and close a chopper. This is quite a large component as compared with the chopper.
In such a chopper-type surface potential sensor, a mechanical actuator (tuning fork-type component) for opening and closing the chopper is required. This disables production of a MEMS (Micro Electro Mechanical Systems) device, making miniaturization of the surface potential sensor difficult and its manufacturing cost high, and also requires an operation to incorporate the component into the surface potential sensor.
With the chopper-type surface potential sensor having a large size and quite a large length as thus described, also in applications of measuring a charging potential of a photosensitive drum in a copying machine, it is impossible to ensure a space inside the copying machine which is large enough for moving the surface potential sensor along a width direction of the photosensitive drum. Further, moving the surface potential sensor is not possible since the actuator for driving the chopper would be affected. For that reason, this surface potential sensor can only monitor a portion of an outer periphery of the photosensitive drum, and thus has low reliability in terms of sensing deterioration in the photosensitive drum. On the other hand, in a copying machine for business purpose and the like, inspecting the entire photosensitive drum has been desired due to a requirement for printing quality comparable to that of a letterpress printing machine.
(Electrostatic Induction-Type Surface Potential Sensor)
There is another surface potential sensor using electrostatic force that operates between itself and a charged object. Such a sensor, for example, is disclosed in Patent Document 2. In this surface potential sensor, a movable electrode plate is located inside a frame-shaped support, and the movable electrode plate is supported by a pair of beam portions in a cantilever condition. The beam portion is provided with a strain gauge for detecting a warping quantity of the beam portion.
When the movable electrode plate is made opposed to the charged object, the movable electrode plate is attracted to the charged object due to electrostatic absorption force that operates therebetween. A warping quantity of the beam portion at that time is sensed by the strain gauge, and a potential of the charged object is calculated based on the warping quantity of the beam portion measured by the strain gauge.
However, in the surface potential sensor disclosed in Patent Document 2, it is just that the movable electrode plate is absorbed by charges induced on the surface of the movable electrode plate due to electrostatic induction, and hence the electrostatic absorption force is small. For this reason, when the beam portion has high rigidity, displacement in movable electrode plate is small, and it is thus difficult to obtain high measurement sensitivity. Further, when the beam portion has low rigidity and displacement in movable electrode plate is large, inclination of the movable electrode plate becomes large at the time of displacement. This may cause a characteristic change of the surface potential sensor depending on its installed direction, and hence it is difficult to incorporate the surface potential sensor into equipment. Moreover, since the surface potential sensor is greatly affected by acceleration due to vibration, it is also difficult to perform accurate measurement. Furthermore, in such a surface potential sensor, the movable electrode plate is attracted to the charged object regardless of the polarity (positive or negative voltage) of the charged object, and hence it is not possible to find the polarity of the charged object.